A hot-rolled steel sheet is manufactured in the following way: a slab heated in a heating furnace is roughly rolled in a roughing mill to become a roughly rolled material (hereinafter sometimes referred to as a “rough bar”); thereafter, the rough bar, which has been transported to a finishing mill by a transporting table, is rolled into a predetermined size in the finishing mill; then after going through a cooling process in which it is cooled in predetermined conditions, it is finally coiled by a coiler.
Among the hot-rolled steel sheets manufactured in this manner, a steel sheet for automobiles, structural materials and the like is required to have excellent mechanical properties such as strength, workability, and toughness. In order to enhance these mechanical properties comprehensively, it is effective to refine the structure of the steel sheet, and therefore a number of methods for obtaining a steel sheet with a fine structure have been sought. Further, if the structure of the steel sheet is refined, it is possible to obtain a high-strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloy elements to be added is reduced.
As a method of refining the structure of the steel sheet, it is known that large reduction rolling is carried out especially in the later stage of finish rolling to refine austenite grains and to accumulate rolling strains in the steel sheet, thereby obtaining fine ferrite grains after finish rolling. The finishing mill is constituted by a plurality of stands, and the steel sheet accumulates strains inside by being rolled; however, since the strains are released as time passes, it is desirable to roll the steel sheet within a short period of time in order to accumulate the rolling strains. Further, in view of inhibiting recrystallization or recovery of the austenite grains and facilitating the ferrite transformation, it is effective to rapidly cool the steel sheet to a temperature of 600° C. to 750° C. as quickly as possible after the finish rolling. In addition, in order to enable uniformity of the mechanical properties of the steel sheet, the ferrite grains need to be in the same predetermined grain size, and the temperatures of the steel sheet at a time of starting rapid cooling and at a time of completing the rapid cooling need to be strictly controlled to predetermined temperatures.
On the other hand, when a steel sheet is rolled, it will be oxidized by oxygen in the air, and oxidized scales will be formed on the surface of the steel sheet. The oxidized scales formed are removed by a descaler which is arranged on an entry side of the finishing mill. However, if they are not removed enough, the cooling properties at a time of rapid cooling after rolling will vary between the area where the oxidized scales have been removed and the area where they remain. Accordingly, the temperature of the steel sheet cannot be controlled strictly, causing the mechanical properties thereof to deteriorate. Furthermore, the surface properties of the hot-rolled steel sheet as a finished product also deteriorate.
Therefore, in order to manufacture a hot-rolled steel sheet having excellent mechanical properties and favorable surface properties, it is necessary to fully remove the oxidized scales. At a time of removing the oxidized scales by spraying high-pressure water at the steel sheet or removing them by a descaler (a descaling device), if the oxidized scales are too thin, they cannot be removed well. So in order to make the oxidized scales grow thick so that they can be easily removed, it is necessary to heat the rough bar up to a predetermined temperature and to facilitate growth of the oxidized scales.
For example, in order to manufacture a high tensile steel sheet having both strength and workability, it is effective to add Si to the composition of the steel sheet. However, when the steel sheet contains Si, an oxide mainly composed of iron and Si will be produced at a boundary between the base material and the oxidized scales. A melting point of this oxide is approximately 1100° C. When the oxide is in a solid state, it interrupts transfer of iron ions supplied from the base material that is necessary for the oxidized scales to grow, thus preventing the oxidized scales from growing thick. Therefore, when the steel sheet contains Si, heating the rough bar to 1100° C. or more will cause the oxide mainly composed of iron and Si to melt, enabling iron ions to be supplied and the oxidized scales to grow thick. As a result, the oxidized scales can be easily removed by the descaler.
On the contrary, when this oxide does not melt and the oxidized scales remain thin, the oxidized scales that cannot be removed by the descaler will remain on the surface of the steel sheet and will be further oxidized by oxygen in the atmosphere, to turn from ferrous oxide to red ferric oxide. This ferric oxide not only changes the aforementioned cooling properties at the time of rapid cooling, but also largely changes emissivity of the surface of the steel sheet when it remains on the steel sheet, thus causing errors in measurement values obtained by a radiation thermometer. As such, when the oxide mainly composed of iron and Si does not melt, not only will it be extremely difficult to strictly control the temperature of the hot-rolled steel sheet, but also problems will arise in the quality control.
As the technique related to a manufacturing method and a manufacturing apparatus of such a hot-rolled steel sheet, Patent Document 1 for example discloses a manufacturing facility of a steel strip comprising in the following mentioned order: rough machining equipment which subjects a hot slab to single or multiple pass reduction in the sheet thickness direction to obtain a rough bar; first rapid cooling equipment which is arranged directly near an exit side of the rough machining equipment and cools the rough bar; coilbox equipment which coils the cooled rough bar; rapid heating equipment which heats the coiled rough bar while uncoiling it; and finishing equipment which gives rolling reduction in the sheet thickness direction to the heated rough bar to obtain a steel strip. Further, Patent Document 1 discloses a manufacturing method of a steel strip using such a manufacturing facility of a steel strip; and the technique described in Patent Document 1 aims to manufacture a steel strip having an ultra fine grain structure. In addition, Patent Document 2, with an aim to manufacture a steel sheet having excellent mechanical properties and surface properties, discloses a manufacturing method of a hot rolled steel sheet wherein in manufacturing a hot-rolled steel sheet by roughly rolling a heated steel slab with a roughing mill to obtain a rough bar and carrying out finish rolling with a finishing mill while controlling the temperature of the steel sheet on an exit side of the finishing mill to a target value by heating the rough bar with a heating device arranged on an entry side of the finishing mill comprising a plurality of stands and/or by cooling the rough bar with a cooling device arranged in at least one interstand space among the plurality of stands, the temperature of the rough bar on the entry side of the finishing mill is predicted before starting the rough rolling, based on a predicted value of the temperature of the rough bar on the exit side of the roughing mill; and a preset value of a rolling speed of finish rolling and/or a preset value of a thickness of the rough bar are corrected and set such that the predicted value of the temperature of the rough bar on the entry side of the finishing mill becomes a target value or above. Further, Patent Document 2 discloses a manufacturing apparatus of a hot-rolled steel sheet to which this manufacturing method can be applied.